In general, the degree of purity of various liquid media can be determined by measuring the conductivity or resistivity of a given liquid medium, since the resistivity or conductivity of the liquid medium typically varies in some known relationship with the level of impurities (e.g., total dissolved solids) that are present in the liquid medium. For example, the resistivity of ultrapure water at 25° C. is taken to be 18.18 MΩ-cm. As the concentration of various ions in the water increases, the ability of the water to conduct an electrical current increases and the resistivity decreases from that value. Conversely expressed, the conductivity of water—which is the inverse of its resistivity—will increase from a value of 5.5 μS/m. Thus, it is common to measure resistivity, and hence purity, of a liquid medium such as water by disposing a pair of spaced-apart electrodes into the liquid medium such that the medium fills the gap between the electrodes and measuring electrical resistance or conductivity between the electrodes as provided by the liquid medium.
In various industrial processes, the purity of a given process stream (e.g., a supply of ultrapure water for use in manufacturing pharmaceuticals, semiconductor devices, etc.) may be measured on a continuous basis, and a sensing probe 10, such as a “SMART SENSOR” sensing probe, available from Mettler-Toledo Ingold Inc. (Thornton) in Bedford, Mass., and shown in FIG. 1, can be used to do so. The sensing probe 10 includes a sensing head 12; a conductor cable 14 extending from the sensing head 12; and an output connector 16 located at an opposite end of the conductor cable 14. The sensing head 12 includes, among other sensing components, a pair of electrodes that are spaced apart from each other. For example, the SMART SENSOR sensing probe 10 shown in FIG. 1 has a longitudinally extending, central positive electrode, not shown, which is surrounded by a longitudinally extending negative shell electrode 18 in coaxial relation to the central electrode. A threaded collar 20 at the base of the sensing head 12 allows the sensing head 12 to be screwed into a probe gland that is attached to a conduit that conveys a given process stream liquid, with the central and shell electrodes extending into the process stream liquid. Additionally, a sensing probe 10, such as the SMART SENSOR sensing probe, may also include a temperature sensor to measure the local temperature of the liquid medium into which the sensing probe extends, as well as sensors to measure other parameters associated with the process stream (e.g., flow rate, ORP (oxidation reduction potential), etc.) on a continuous basis. Furthermore, the output connector 16 of the sensing probe 10 is connected to a monitoring instrument, not illustrated, such as a 770PC multiparameter instrument/transmitter, also available from Mettler-Toledo Ingold Inc. (Thornton) in Bedford, Mass. The monitoring instrument “reads” the signals from the sensing probe 10 and displays them to an operator as parameter values and/or otherwise provides the signals and/or the corresponding parameter values to a process control center.
Further still, a sensing probe 10 used to measure liquid purity by means of resistivity/conductivity has a cell constant associated with it. The cell constant, which varies with the geometry of the sensor electrodes, expresses the relationship (i.e., a ratio) between resistivity as measured by a given sensing probe 10 and the actual resistivity of the liquid medium. Therefore, in order to convert measured resistivity into actual resistivity—and hence purity—of the sampled liquid, it is necessary to know the cell constant of a given sensing probe 10. A SMART SENSOR sensing probe has an electrically-erasable programmable read-only memory (EEPROM) in which the associated cell constant is stored. The cell constant is transmitted automatically to a 770PC multiparameter instrument/transmitter when the sensing probe is connected to the process controller.
Over time, however, a sensing probe 10 can “go bad” and no longer provide accurate sensing results. For example, if the geometry of the electrodes varies for some reason such as thermal expansion and contraction without the electrodes returning to their original configuration, then the cell constant will “drift.” If that geometric variation is nominal, then the sensing probe can be recalibrated, i.e., its cell constant can be determined anew (and in the case of a SMART SENSOR sensing probe, reprogrammed into the EEPROM). On the other hand, if the geometry of the electrodes varies by an excessive amount such that the cell constant changes by an amount greater than ±1 percent, then the sensing probe will be unusable and should be discarded.
Conventionally, it is known to use a calibration test solution of fixed, known concentration/degree of purity to calibrate a sensing probe. But, if the degree of purity of the calibration solution is not known, it may be possible to ascertain the sensing probe's cell constant by varying the temperature of the calibration solution and monitoring how the measured resistivity changes. This is because resistivity varies with temperature and concentration/purity of the calibration solution will “drop out of the analysis” (because it is held constant from calibration test point to calibration test point). Because calibration test solutions can be contaminated easily, however, it is possible that the concentration/purity of the test solution is not what it is believed to be. Alternatively, even if a protocol that does not require knowledge of the test solution's concentration/purity is used, such a protocol is generally more complex than one using a calibration test solution in which the concentration/purity of the test solution is (putatively) known, and therefore is subject to its own types of errors.
Accordingly, there is a need in the art for a device and methodology for calibrating and/or verifying the integrity of resistivity-based sensing probes that is less complex and/or more reliable than such prior art calibration methodologies.